The present invention relates to rolling bearings for use in various types of motors for general industrial applications, and in particular those designed to improve the cage noise under low temperature conditions, and to decrease fretting damages (abrasion) and bearing torque.
The driving motor unit used for an air conditioning apparatus (which will be called air conditioner hereinafter) is a typical example of general-purpose motors. Air conditioners are recently provided with ever-improved performance and multi-functions; for example, with the aid of inverter control, they first lower the room temperature very quickly in a short time by a high-speed operation, and then keep the room temperature constant by a slow-speed operation. Accompanying this, a low noise operation is required in which the blowing-out sound as well as the rotating sound of the motor is well suppressed. However, due to the decrease of the cooling efficiency inside the apparatus during the slow-speed operation, the temperature of the rolling bearings installed in the motor sometimes rises to 100 to 120xc2x0 C. Thus, it becomes difficult to secure the appropriate oil film thickness for lubrication, and further the grease packed in the rolling bearings tend to deteriorate. When the deterioration proceeds, noise generates.
For the outdoor unit of an air-conditioner, the initial noise associated with the bearing sound (cage sound) when the conditioner is started to operate under a low temperature condition such as in winter becomes a problem in some cases.
Furthermore, the motor units of air-conditioners with rolling bearings installed therein are sometimes transported over long distances from a motor manufacturer to end users by a truck. In such a long distance transportation, the truck traces the fine unevenness of roads, and the associated movement is transmitted to the rolling bearing in the form of repetitive impact loads, which cause the rolling element of the rolling bearing to repeatedly put into microscopic contact with the raceway surface. Such microscopic contacts sometimes cause a fretting damage (abrasion) on the raceway surface, and the damage acts as a source of noise.
On the other hand, air-conditioners are being made more compact in size and lower in power to decrease the heat generated by the motor by taking into consideration environmental regulations in addition to the trend of high performance as well as multi-function. Accordingly, for the rolling bearing for these uses, torque characteristics are regarded as one of the most significant features. The dynamic frictional torque of a rolling bearing is caused by the friction due to the microscopic sliding of the rolling contact surface, the sliding friction at the sliding contact part in the bearing and the viscous resistance of the grease. Among these causes, the viscous resistance of the grease is known to depend on the kinematic viscosity of the base oil and the penetration of the grease. Since the kinematic viscosity of the base oil is associated with the shear resistance of the oil when a fluid lubricating film is formed, the reduction of kinematic viscosity is an important role for the reduction of the dynamic frictional torque of the rolling bearing. Further, since grease penetration is related to the channeling behavior when the grease is subjected to a shear force in the bearing during the rotation of bearing, the decrease of the penetration is also quite effective.
However, when the kinematic viscosity of the base oil is reduced, it becomes difficult to secure an appropriate thickness of the oil film since the motor for air-conditioners is driven at a relatively slow speed by invertor control as described hereinabove. Moreover, those oils, which have a low kinematic viscosity, generally exhibit a low heat resistance, which suffers from a problem in acoustic durability. On the other hand, the reduction of grease penetration demands the increase in the content of the thickener whereby the quantity of the base oil in the grease relatively reduces. These changes raise the resistance of the grease to mechanical shearing forces, resulting in the reduction of the quantity of the base oil supplied to the lubricating surface of the bearing. Therefore, it becomes impossible to stably secure the lubricating capability over a long period of usage.
It is thus evident that the reduction of the kinematic viscosity of the base oil and the reduction of grease penetration suffer from practical restrictions; for rolling bearings used for the above-cited applications, a suitable range of the kinematic viscosity at 40xc2x0 C. is 10 to 500 mm2/s and the grease penetration is preferably of NLGI No. 2 to 3 grade. In other word, 5 to 20% by mass of the content of the thickener is regarded appropriate. In the motor to which the low noise property, that is, the acoustic durability is demanded, a grease is used that contains an ester as the base oil, and the lithium salt of a fatty acid as the thickener. The ester oil exhibits a better heat resistance compared with mineral oils, and has a function of enhancing the acoustic durability due to the presence of polar groups in the molecular structure, since the polar groups are strongly adsorbed onto metal surfaces to improve abrasion resistance. Further in cases where fretting damages (abrasion) is required to reduce, it is well known that use of a relatively high viscosity of base oil capable of forming a stable oil film is effective.
The radii of curvature of the cross-sectional shapes of the inner and outer raceways can be designed in various manners depending on the level of load and rotating speed. But usually the radius of curvature of the inner raceway as well as the outer raceway is made equal to 52% of the diameter of the ball. This is based on the fact that, in Interpretative Table 2 xe2x80x98Radii of raceway grooves and decreasing coefficientxe2x80x99 of xe2x80x9cMethod of Calculating Dynamic Load Rating and Standard Life of Rolling Bearingxe2x80x94Interpretationxe2x80x9d in JIS standard (JIS B 1518-1992), the radius of curvature of the cross-sectional shape is made equal to 52% of the diameter of the rolling element for the calculation of the dynamic load rating of a deep groove ball bearing. Also, in the bearing catalogue published by the present applicant, dynamic and static load rating are calculated with the assumption of the radii of curvature of the cross-sectional shapes of the inner raceway and the outer raceway being 52% of the diameter of the rolling element. As is seen from these facts, the radii of curvature of the cross-sectional shapes of the inner raceway and the outer raceway are usually made equal to 52% of the diameter of the rolling element.
Corresponding to the market trend of making air conditioners more advanced in performance and provided with multi-functions, and also considering environmental regulations, the specifications of the rolling bearing installed in the air conditioners are expected to achieve improved acoustic characteristics, the reduction of fretting damages (abrasion) and a lowered torque. Such needs will probably be more and more urgent and serious in the future.
Accordingly, the invention, which has been devised under such circumstances, has an object of providing rolling bearings that improve in the initial acoustic (case noise) characteristics at the start of operation under low temperature conditions, and that are designed to achieve reduced fretting damages (abrasion) and a reduced bearing torque.
To achieve the object, the invention provides the following rolling bearing.
(1) A rolling bearing comprising an outer ring having on its inner periphery an outer raceway, an inner ring having on its outer periphery an inner raceway, plural rolling elements arranged between said outer raceway and said inner raceway in a freely rotatable manner, a cage holding said plural rotatable elements in a freely rotatable condition, and a grease packed in the space between said outer raceway and said inner raceway, said grease comprising a base oil composed of a lubricant having a polar group in its molecular structure and a non-polar lubricant, and a metallic soap-based thickener containing a long-fiber material whose major axis part length is at least 3 xcexcm, and the radius of curvature of the cross-sectional shape of said inner raceway and the radius of curvature of the cross-sectional shape of said outer raceway being in the range of from 51.0% to smaller than 60.0% of the diameter of said rolling element.
(2) A rolling bearing comprising an outer ring having on its inner periphery an outer raceway, an inner ring having on its outer periphery an inner raceway, plural rolling elements arranged between said outer raceway and said inner raceway in a freely rotatable manner, a cage holding said plural rotating elements in a freely rotatable condition, and a grease packed in the space between said outer raceway and said inner raceway, said grease comprising a base oil composed of a lubricant having a polar group in its molecular structure and a non-polar lubricant, and a metallic soap-based thickener containing a long-fiber material whose major axis part length is at least 3 xcexcm, and the radius of curvature of the cross-sectional shape of said inner raceway being in the range of from 50.5% to 56% of the diameter of said rolling element, and the radius of curvature of the cross-sectional shape of said outer raceway being in the range of from 53.0% to 58.0% of the diameter of said rolling element.
In the rolling bearing of the invention, bearing torque can be reduced due to the use of the above-specified grease composition. The long-fiber material having a major axis part length of at least 3 xcexcm contained in the thickener for the grease composition is oriented by the shearing force exerted by the rotation of the bearing, and acts to reduce the bearing torque. This bearing torque-reducing effect is further enhanced by jointly using a non-polar lubricant in the base oil. Further, the lubricant having a polar group in its molecular structure (which will be called polar group-containing lubricant hereinafter) contained in the base oil plays a role similar to that of the conventional polar group-containing base oil (such as ester oils). The polar group is preferentially adsorbed on the contact surface for the rotary part of the bearing to form an adsorption film, which reduces the bearing torque through the improvement of surface frictional property. Still further, the polar group-containing lubricant interacts with the long-fiber material comprising the micelle structure of the metallic soap to reduce the bonding force between the long-fiber materials. Hence, the shear resistance of the grease during bearing rotation is reduced, resulting in a further reduction of bearing torque. Because of such mechanisms, even if the radius of curvature of the cross-sectional shape of the inner raceway or the outer raceway is made smaller than 52% of the diameter of the rolling element, an improved dynamic torque property is achieved compared to the property achieved in conventional specifications provided that the radius of curvature is at least 51% of the diameter of the rolling element.
By making the radius of curvature of the cross-sectional shape of the inner raceway or the outer raceway 51.0% to smaller than 60.0% of the diameter of the rolling element, or by making the radius of curvature of the cross-sectional shape of the inner raceway 50.5% to 56% of the diameter of the rolling element, and the radius of curvature of the cross-sectional shape of the outer raceway 53.0% to 58.0% of the diameter of the rolling element, the elastic deformation of the contact portion in the rolling surface of the rolling element with the inner raceway surface or the outer raceway surface is reduced. In other words, the Hertian contact ellipse becomes smaller, thus the differential slippage is reduced, resulting in the reduction of bearing torque. Simultaneously, against the impact loading repetitively applied during transportation, fretting damages (abrasion) can be reduced which occur at the inner raceway surface or the outer raceway surface, leading to the improvement of acoustic durability. With a radius of curvature of the cross-sectional shape of the inner raceway or the outer raceway larger than the upper limit mentioned above, the maximum Hertian contact pressure at the contact elliptic area becomes too large, thus shortening the rolling fatigue life of the inner raceway or the outer raceway. Hence, an unfavorable acoustic property and a short flaking life result. By making the radius of curvature of the cross-sectional shape of the outer raceway larger than the radius of curvature of the cross-sectional shape of the inner raceway, the contact surface pressure between the rolling surface of the ball and the inner raceway or the outer raceway can be decreased.
Since the rolling bearing of the invention can reduce the friction acting between the raceway surface of the rolling element and the outer raceway and the inner raceway during the relative revolution of the outer ring and inner ring, heat generation inside the rolling bearing can be effectively suppressed. As a result, the deterioration of the grease composition packed in the bearing is prevented to secure improved acoustic durability for the rolling bearing.